The issue is that your input file uses commas as the decimal separator, rather than a period ("decimal point"). This means that MATLAB is treating those elements a text rather than numbers.
I did a global replacement of commas for periods, and then your file loads the data as numbers.
T15 = readtable('73_4.csv');
I don't think there is a way to change MATLAB's requirement of using a decimal point for input. So, I think that doing the replacement in the input file is your only option. (But I could be wrong.)